1-http://www.usgs.gov/. [Online]
2- http://en.wikipedia.org/wiki/United_States_Geological_Survey. [Online]
3-http://nationalmap.gov/ustopo/. [Online]
4- http://www.ordnancesurvey.co.uk/oswebsite/. [Online]
5- Digital Photogrammetry, Developments at Ordnance Survey. Lynne Allan, DavidHolland. Amsterdam : International Archives of Photogrammetry and Remote Sensing, 2000. Vols. XXXIII, Part B. 2